A Hidden World Thrives Below the Snow

“The winter is a pretty incredible time of year,” Jonathan Pauli told me. Looking out the window of his office at Wisconsin’s stubborn crust of snow, it occurred to me that “incredible” might not be the most popular adjective, especially this year, as a harrowing winter slowly releases its grip on the United States.

Pauli and his colleague Benjamin Zuckerberg explained that winter’s thick blanket of snow,inhospitable as it looks, actually provides a safe haven for plants and animals that spend their lives in northerly latitudes. “You look at the landscape and you see a blanket of homogeneous white,” said Pauli. “You think that it makes everything simple. But what is kind of fun to think about is that complex world that lives underneath the snow.”

Pauli and Zuckerberg are both professors in the Department of Forest and Wildlife Ecology at the University of Wisconsin-Madison. They call the narrow band of stability that exists between snow and soil the “subnivium.” In this wintry stratum, plants can germinate and photosynthesize and animals stay active -- some, like shrews, even raise litters here. But according to the scientists’ research, the subnivium is endangered.

Two key features help the subnivium support its tenants: temperature stability and air pockets that give animals a little breathing room.

Stable temperatures -- even low ones -- are easier than fluctuating ones for organisms to accommodate. Because water in any form takes a long time to heat up or cool down, snow is a great insulator. It traps the heat rising from the soil so that even when the air temperature plummets, the temperature at the bottom of a column of snow will hover right around freezing. Meanwhile, the heat emanating from the soil creates a rising cloud of water vapor. Those water molecules condense as they move upward, creating a band of loose, icy snow where animals can move around easily.

In this stable, humid ecosystem, spiders, woolly bear caterpillars, wood frogs, voles, shrews, lawn-killing fungi called “snow molds,” and delicate glacier lilies all thrive. As the climate changes, though, they might not be thriving for much longer.

But doesn’t it seem like winter would be the one time when a warming climate could be a boon for wildlife? At first blush, Pauli admitted, “it does seem paradoxical. Winter is a period of resource limitations, and if it gets warmer, that’s got to be a good thing!”

Like snow cover, though, it isn’t as simple as it looks. Michael Notaro, a climate researcher at UW-Madison, explained that by the end of the century the 6- to 12-degree jump predicted for the upper Midwest will mean winters that are less snowy. And the snow that does fall won’t be the few dry, fluffy inches at a time that create the most stable subnivium.

Warm air holds more water than cold air, so when the conditions are right for precipitation, “there’s more moisture to wring out,” Notaro explained. By the middle of this century Notaro’s models predict that the Midwest will be walloped by big snowstorms that drop wet, heavy snow.

The consequence will be a snowpack that fluctuates both in depth and density. And without a reliably thick blanket of snow, the subnivean temperature starts to fluctuate, tracking the temperature of the air above it. The ironic result is that warmer winters will actually leave these subnivean organisms confronting colder temperatures.

That’s a big problem. The subnivium’s inhabitants have developed very finely tuned, if occasionally bizarre, survival strategies that won’t work in new conditions. The wood frog, for example, survives freezing by injecting all its tissues with glucose, which acts as a kind of cellular antifreeze, and then draining all the blood from its organs until the spring thaw jump-starts the frog’s heart.

But below about 18 degrees Fahrenheit, the extra glucose can’t keep ice from invading the frog’s tissues, tearing apart its cells. Even if the temperature never dips below that critical point, freeze/thaw cycles driven by newly unstable temperatures could leave the frog too exhausted to make it though the winter. Other animals’ adaptive strategies are just as delicate.

“It’s fascinating to think about how animals make a living,” Pauli said. “I mean, it really is fascinating. We crawl into our 65-degree houses and it’s comfy and cozy, but these organisms are making a living in a hostile environment.” The subnivium has been a refuge from that hostility, but Pauli believes that climate change “might simply outstrip the ability of organisms to make a living there.”

Until now this problem has been largely overlooked, partly because the subnivium itself is tucked away. Zuckerberg explained that “degradation in that environment can be pretty invisible to the naked eye.” Unlike a stripped rainforest or garbage-choked river, a shallow, dense band of snow looks a lot like a thick, loose one, but only the latter fosters a healthy subnivium.

Climate models predict the biggest temperature jumps during the coldest months of the year. “Most ecology labs are kind of hibernating during the winter,” Zuckerberg says. But neither Zuckerberg nor Pauli will be sleeping their days away: they’ll be out there in that incredible winter, looking for everything hidden under the snow.

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